1. Field of the Invention
The present invention relates to a display device for displaying an image by light emitting elements disposed in pixels, a method of driving the same, and an electronic unit having such a display device.
2. Description of the Related Art
In recent years, in the field of a display device for displaying an image, a display device (organic EL (Electro Luminance) display device) using, as a light emitting element, an optical element of a current driving type whose light emission luminance changes according to the value of a flowing current, for example, an organic EL element is developed and is being commercialized.
An organic EL element is a self light emitting element different from a liquid crystal element or the like. Consequently, in an organic EL display device, a light source (backlight) is unnecessary. As compared with a liquid crystal display device requiring a light source, visibility of an image is higher, power consumption is lower, and response of the element is faster.
As driving methods of an organic EL display device, like driving methods of a liquid crystal display device, there are a simple (passive) matrix method and an active matrix method. The former method has, although the structure is simple, a problem such that it is difficult to realize a large-size and high-resolution display device. Consequently, at present, an active matrix method as the latter method is actively developed. In the method, current flowing in organic EL elements disposed for pixels is controlled by an active element (generally, TFT (Thin Film Transistor)) provided in a drive circuit arranged for each of the organic EL elements.
It is generally known that the current-voltage (I-V) characteristic of an organic EL element deteriorates with lapse of time. In a pixel circuit for current-driving an organic EL element, when the I-V characteristic of the organic EL element changes with time, the value of current flowing in a drive transistor changes. Consequently, the value of current flowing in the organic EL element itself also varies, and the light emission luminance also changes accordingly.
There is a case that a threshold voltage Vth and mobility μ of the drive transistor change with time, or vary among pixel circuits due to variations in manufacturing processes. In the case where the threshold voltage Vth and the mobility μ of the drive transistor vary among pixel circuits, the value of current flowing in the drive transistor varies among pixel circuits. Consequently, even when the same voltage is applied to the gate of the drive transistor, the light emission luminance of the organic EL element varies, and uniformity of a screen deteriorates.
There is consequently a proposal for maintaining the light emission luminance of an organic EL element constant without being influenced by the variations, even when the I-V characteristic of the organic EL element changes with time, a threshold voltage Vth and mobility μ of a drive transistor changes with time or varies among pixel circuits. Concretely, a display device is proposed, having both a function of compensating fluctuations in the I-V characteristic of an organic EL element and a function of correcting fluctuations in the threshold voltage Vth and mobility μ of a drive transistor (refer to, for example, Japanese Unexamined Patent Application Publication No. 2008-33193).